Grain oriented electrical steel sheet and method of manufacturing the same

ABSTRACT

A method of manufacturing a grain oriented electrical steel sheet includes subjecting a slab to rolling to a final sheet thickness; subjecting the sheet to subsequent decarburization; applying an annealing separator composed mainly of MgO to a surface of the sheet before subjecting the sheet to final annealing; subjecting the sheet to subsequent tension coating; and subjecting, after the final annealing or the tension coating, the sheet to magnetic domain refining treatment by electron beam irradiation, wherein a degree of vacuum during the electron beam irradiation is 0.1 to 5 Pa, and a tension to be exerted on the steel sheet during flattening annealing is 5 to 15 MPa.

RELATED APPLICATIONS

This is a divisional of U.S. application Ser. No. 13/814,357, filed Feb.5, 2013, which is a §371 of International Application No.PCT/JP2011/004410, with an international filing date of Aug. 3, 2011 (WO2012/017655A1, published Feb. 9, 2012), which is based on JapanesePatent Application No. 2010-177619 filed Aug. 6, 2010.

TECHNICAL FIELD

This disclosure relates to a grain oriented electrical steel sheetpreferably used for iron core materials such as transformers, and amethod of manufacturing the same.

BACKGROUND

Grain oriented electrical steel sheets, which are mainly used as ironcores of transformers, are required to have excellent magneticproperties, in particular, less iron loss.

To meet this requirement, it is important that secondary recrystallizedgrains are highly aligned in the steel sheet in the (110)[001]orientation (or so-called the Goss orientation) and impurities in theproduct steel sheet are reduced. Additionally, there are limitations tocontrol crystal orientation and reduce impurities in terms of balancingwith manufacturing cost, and so on. Therefore, some techniques have beendeveloped to introduce non-uniformity to the surfaces of a steel sheetin a physical manner and reducing the magnetic domain width for lessiron loss, namely, magnetic domain refining techniques.

For example, JP 57-002252B proposes a technique for reducing iron lossof a steel sheet by irradiating a final product steel sheet with laser,introducing a high dislocation density region to the surface layer ofthe steel sheet and reducing the magnetic domain width. JP 06-072266Bproposes a technique to control the magnetic domain width by electronbeam irradiation.

However, when a grain oriented electrical steel sheet that has beensubjected to the above-mentioned magnetic domain refining treatment isassembled into an actual transformer, it may produce significant noise.

It could therefore be helpful to provide a grain oriented electricalsteel sheet that may exhibit excellent low noise and low iron lossproperties when assembled as an actual transformer, along with anadvantageous method of manufacturing the same.

SUMMARY

We thus provide:

-   -   [1] A grain oriented electrical steel sheet comprising a        forsterite film formed on a surface thereof, being subjected to        strain introduction by electron beam and having a magnetic flux        density B8 of 1.92 T or higher,        -   wherein a ratio (Wa/Wb) of a film thickness of the            forsterite film on a strain-introduced side of the steel            sheet (Wa) to a film thickness of the forsterite film on a            non-strain-introduced side of the steel sheet (Wb) is 0.5 or            higher, and        -   wherein a magnetic domain discontinuous portions in a            surface of the steel sheet on the strain-introduced side has            an average width of 150 to 300 μm, and a magnetic domain            discontinuous portion in a surface of the steel sheet on the            non-strain-introduced side has an average width of 250 to            500 μm.    -   [2] A method of manufacturing a grain oriented electrical steel        sheet, the method comprising:        -   subjecting a slab for a grain oriented electrical steel            sheet to rolling to be finished to a final sheet thickness;        -   subjecting the sheet to subsequent decarburization;        -   then applying an annealing separator composed mainly of MgO            to a surface of the sheet before subjecting the sheet to            final annealing;        -   subjecting the sheet to subsequent tension coating; and        -   subjecting, after the final annealing or the tension            coating, the sheet to magnetic domain refining treatment by            electron beam irradiation, wherein        -   (1) the degree of vacuum during the electron beam            irradiation is 0.1 to 5 Pa, and        -   (2) a tension to be exerted on the steel sheet during            flattening annealing is controlled at 5 to 15 MPa.    -   [3] The method of manufacturing a grain oriented electrical        steel sheet according to item [2] above, wherein the slab for        the grain oriented electrical steel sheet is subjected to hot        rolling, and optionally, hot rolled sheet annealing, and        subsequently subjected to cold rolling once, or twice or more        with intermediate annealing performed therebetween, to be        finished to a final sheet thickness.

It is possible to provide a grain oriented electrical steel sheet thatallows an actual transformer assembled therefrom to effectively maintainthe effect of reducing iron loss by magnetic domain refinement using anelectron beam. Therefore, the actual transformer may exhibit excellentlow noise properties, while maintaining excellent low iron lossproperties.

BRIEF DESCRIPTION OF THE DRAWINGS

Our steel sheets will be further described below with reference to theaccompanying drawings, wherein:

FIG. 1 illustrates a cross-section for measuring the thickness of aforsterite film.

FIG. 2 illustrates the result of observing magnetic domains of the steelsheet.

DETAILED DESCRIPTION

We analyzed the cause of increases in noise when using a grain orientedelectrical steel sheet subjected to magnetic domain refinement treatmentas an actual transformer. As a result, we found that an increase intransformer noise is caused by a reduction in the thickness of aforsterite film (a film composed mainly of Mg2SiO4) in thestrain-introduced portion when thermal strain is introduced for magneticdomain refinement. In this respect, we also found that noise degradationcan be prevented by appropriately adjusting a ratio of a film thicknessof the forsterite film on a strain-introduced side of the steel sheet(Wa) to a film thickness of the forsterite film on anon-strain-introduced side of the steel sheet (Wb).

Further, we found that both the average width of a magnetic domaindiscontinuous portion in a surface of the steel sheet on thestrain-introduced side and the average width of a magnetic domaindiscontinuous portion in a surface of the steel sheet on thenon-strain-introduced side have to be adjusted within a proper range. Asused herein, the term strain-introduced side indicates the side on whichelectron beam has been irradiated, and non-strain-introduced side refersto the side on which electron beam has not been irradiated.

Our steel sheets and methods will be specifically described below.

One of the measures to be taken to mitigate an increase in noise whenusing a grain oriented electrical steel sheet as an actual transformerthat has been subjected to strain application and magnetic domainrefinement treatment is to satisfy all of the three points given below.

Control of the Thickness of a Forsterite Film on the Strain-IntroducedSide

The first point is control of the thickness of a forsterite film wherestrain is introduced. Control of the thickness of a forsterite film isimportant for the reasons explained below.

A forsterite film on a surface of the steel sheet applies tension to thesteel sheet. A variation in the thickness of this forsterite film leadsto a non-uniform tension distribution of the steel sheet. Thenon-uniform tension distribution results in a distortion in themagnetostrictive vibration waveform of the steel sheet which causes anincrease in noise. As a result, noise increases with a superimposedharmonic component. Accordingly, to mitigate this increase in noise, itis important to mitigate a reduction in the thickness of the forsteritefilm at the time of introduction of thermal strain. That is, a ratio(Wa/Wb) of a film thickness of the forsterite film on astrain-introduced side (Wa) to a film thickness of the forsterite filmon a non-strain-introduced side (Wb) should be 0.5 or higher, preferably0.7 or higher.

Besides, the thickness of the forsterite film on each side of the steelsheet before the introduction of strain is usually the same. Thus, themaximum value of Wa/Wb is about 1.

FIG. 1 is a schematic diagram illustrating a cross-section of a steelsheet having a forsterite film. The forsterite film appears to benon-uniform in thickness and has significant irregularities as viewed ona short periodic basis. However, the thickness of the forsterite filmmay be determined from an average of thickness measurements by using asufficiently large measurement distance. Specifically, the thickness ofthe forsterite film may be determined by cutting a sample from across-section of the steel sheet, determining an area of the forsteritefilm over a predetermined measurement distance (preferably 1 mm) (using,preferably, SEM observation and image analysis), and calculating anaverage of thickness measurements of the film on that surface.

To satisfy the above-described ratio (Wa/Wb), it is important tomitigate a reduction in the thickness of the forsterite film wherethermal strain is applied as mentioned above. Means for mitigating thisreduction will be described below. Above all, it is important to form agood forsterite film. As used herein, a good forsterite film means aforsterite film that has fewer gaps due to cracking and thus is highlydensified. In addition, what is the most influential factor among thosecausing damage such as cracking to the forsterite film is the tension tobe applied to the steel sheet during flattening annealing. If thistension is strong, the forsterite film is damaged and cracking occurs,for example. Thus, in an annealing furnace where the steel sheet hashigh temperature and thus is more sensitive to tension, it is necessaryto control the tension at 15 MPa (1.5 kgf/mm2) or lower.

On the other hand, we control the above-described tension to be 5 MPa(0.5 kgf/mm2) or higher. This is because the tension of less than 5 MParesults in inadequate shape correction of the steel sheet. In addition,it is necessary to control the degree of vacuum during electron beamirradiation. It is generally believed that a higher degree of vacuum isbetter for electron beam irradiation. However, we found that allowing anadequate amount of oxygen to be left during electron beam irradiation iseffective in mitigating reduction of the forsterite film. While themechanism for this has not been clarified, we believe as follows:oxidation of the steel sheet due to the residual oxygen at the time ofintroduction of thermal strain might have some influence on maintenanceof the film thickness of the forsterite film. To mitigate a reduction inthe film thickness of the forsterite film, the degree of vacuum is 0.1to 5 Pa. If the degree of vacuum is below 0.1 Pa, it is not possible tomitigate the reduction of the forsterite film. Alternatively, if thedegree of vacuum is above 5 Pa, it is not possible to apply thermalstrain to the steel sheet in an effective manner. The degree of vacuumis more preferably 0.5 to 3 Pa. Control of magnetic domain discontinuousportions in a surface of the steel sheet on the strain-introduced sideand in a surface of the steel sheet on the non-strain-introduced side

The second point is control of magnetic domain discontinuous portions ina surface of the steel sheet on the strain-introduced side and in asurface of the steel sheet on the non-strain-introduced side,respectively.

While this control of the thickness of the forsterite film may somewhatmitigate an increase in noise, an actual transformer is required toexhibit even lower noise properties and still lower iron lossproperties.

In other words, to reduce the iron loss of a transformer, it is alsoimportant to reduce the iron loss of the material. That is, to make fulluse of the magnetic domain refinement effect in the material, thefollowing are important:

-   -   (i) To introduce strain until magnetic domain discontinuous        portions are also produced in a surface of the steel sheet on        the strain-introduced side and in a surface of the steel sheet        on the non-strain-introduced side, respectively; and    -   (ii) To minimize the width of each magnetic domain discontinuous        portion because strain introduction leads to degradation in        hysteresis loss.

The following are specific conditions under which the above items (i)and (ii) are satisfied: an average width of a magnetic domaindiscontinuous portion in a surface of the steel sheet on thestrain-introduced side is 150 to 300 μm; and an average width of amagnetic domain discontinuous portion in a surface of the steel sheet onthe non-strain-introduced side is 250 to 500 μm. That is, we satisfyitem (i) by defining an average width of a magnetic domain discontinuousportion in a surface of the steel sheet on the non-strain-introducedside, and satisfy item (ii) by setting the upper limit of each averagewidth. Further, the lower limit of each average width is also setbecause the magnetic domain refinement effect cannot be obtained for awidth smaller than the lower limit.

It should be noted that if the maximum tension during flatteningannealing and the degree of vacuum during electron beam irradiation arenot satisfied as described earlier in relation to the first point, it isextremely difficult to satisfy the above-described heat-affected widthwithout reducing the thickness of the forsterite film.

It should be noted here that what is important is the average widths ofmagnetic domain discontinuous portions, rather than the averageirradiation width. That is, when heat is introduced to the steel sheet,it is diffused in every direction such as the sheet thickness directionor sheet width direction. Accordingly, each magnetic domaindiscontinuous portion affected by such heat usually tends to be widerthan the irradiation width. Additionally, for the same reason, eachmagnetic domain discontinuous portion on the non-strain-introduced sidehas a width larger than that of each magnetic domain discontinuousportion on the strain-introduced side.

The width of a magnetic domain discontinuous portion may be obtained byvisualizing a magnetic domain structure by the Bitter method usingmagnetic colloid so that discontinuous portions formed by the electronbeam irradiation can be identified (see FIG. 2) and, furthermore, bymeasuring the widths of magnetic domain discontinuous portions over apredetermined measurement distance (preferably 20 mm) to calculate anaverage of the width measurements. FIG. 2 is a schematic diagramillustrating the magnetic domain structure of the grain orientedelectrical steel sheet after the magnetic domain refinement treatment,where main magnetic domains are oriented in the horizontal direction andan electron beam is irradiated in the vertical direction at the centerof the figure at a substantially right angle to the horizontaldirection. A magnetic domain discontinuous portion indicates a regionwhere the structure of main magnetic domain is disrupted by electronbeam irradiation, and that substantially corresponds to a regionaffected by the heat caused by the electron beam irradiation.

High Degree of Alignment of Crystal Grains of the Material with the EasyAxis of Magnetization

The third point is the high degree of alignment of crystal grains of thematerial with the easy axis of magnetization.

Regarding the transformer noise, i.e., magnetostrictive vibration, theoscillation amplitude becomes smaller as the degree of alignment ofcrystal grains of the material with the easy axis of magnetizationbecomes higher. Therefore, for noise reduction, a magnetic flux densityB8, which gives an indication of the degree of alignment of crystalgrains of the material with the easy axis of magnetization, should be1.92 T or higher. In this case, if the magnetic flux density B8 is lessthan 1.92 T, rotational motion of magnetic domains to align parallel tothe excitation magnetic field during the magnetization process causes alarge magnetostriction. This results in an increase in transformernoise. In addition, the higher the degree of crystal grain alignment,the greater the magnetic domain refinement effect. The magnetic fluxdensity B8 should also be 1.92 T or higher in view of iron lossreduction.

The strain introduction process is limited to a method by electron beamthat may reduce damage to the film at a strain-introduced portion. Inthis case, when electron beam irradiation is performed, electron beamshould be irradiated in a direction transverse to the rolling direction,preferably at 60° to 90° to the rolling direction, and the irradiationinterval of the electron beam is preferably about 3 to 15 mm. Inaddition, the electron beam is irradiated in a spot-like or linearfashion under the following conditions: acceleration voltage=10 to 200kV; current=0.1 to 100 mA; and beam diameter=0.01 to 0.5 mm. A preferredbeam diameter is 0.01 to 0.3 mm.

Next, the conditions of manufacturing a grain oriented electrical steelsheet will be specifically described below.

A slab for a grain oriented electrical steel sheet may have any chemicalcomposition that allows for secondary recrystallization.

In addition, if an inhibitor, e.g., an AlN-based inhibitor is used, Aland N may be contained in an appropriate amount, respectively, while ifa MnS/MnSe-based inhibitor is used, Mn and Se and/or S may be containedin an appropriate amount, respectively. Of course, these inhibitors mayalso be used in combination. In this case, preferred contents of Al, N,S and Se are: Al: 0.01 to 0.065 mass %; N: 0.005 to 0.012 mass %; S:0.005 to 0.03 mass %; and Se: 0.005 to 0.03 mass %, respectively.

Further, our methods are applicable to a grain oriented electrical steelsheet having limited contents of Al, N, S and Se without using aninhibitor.

In this case, the amounts of Al, N, S and Se are preferably: Al: 100mass ppm or less: N: 50 mass ppm or less; S: 50 mass ppm or less; andSe: 50 mass ppm or less, respectively.

The basic elements and other optionally added elements of the slab for agrain oriented electrical steel sheet will be specifically describedbelow.

C: 0.08 Mass % or Less

C is added to improve the texture of a hot-rolled sheet. However, Ccontent exceeding 0.08 mass % increases the burden to reduce C contentto 50 mass ppm or less where magnetic aging will not occur during themanufacturing process. Thus, C content is preferably 0.08 mass % orless. Besides, it is not necessary to set up a particular lower limit toC content because secondary recrystallization is enabled by a materialwithout containing C.

Si: 2.0 to 8.0 Mass %

Si is an element that is useful to increase electrical resistance ofsteel and improve iron loss. An Si content of 2.0 mass % or more has aparticularly good effect in reducing iron loss. On the other hand, an Sicontent of 8.0 mass % or less may offer particularly good formabilityand magnetic flux density. Thus, the Si content is preferably 2.0 to 8.0mass %.

Mn: 0.005 to 1.0 Mass %

Mn is an element advantageous in improving hot formability. However, Mncontent less than 0.005 mass % has a less addition effect. On the otherhand, Mn content of 1.0 mass % or less provides a particularly goodmagnetic flux density to the product sheet. Thus, Mn content ispreferably 0.005 to 1.0 mass %.

Further, in addition to the above elements, the slab may also containthe following elements as elements to improve magnetic properties:

-   -   at least one element selected from: Ni: 0.03 to 1.50 mass %; Sn:        0.01 to 1.50 mass %; Sb: 0.005 to 1.50 mass %; Cu: 0.03 to 3.0        mass %; P: 0.03 to 0.50 mass %; Mo: 0.005 to 0.10 mass %; and        Cr: 0.03 to 1.50 mass %.

Ni is an element useful to further improve the texture of a hot-rolledsheet to obtain even more improved magnetic properties. However, an Nicontent of less than 0.03 mass % is less effective in improving magneticproperties, whereas an Ni content of 1.5 mass % or less increases, inparticular, the stability of secondary recrystallization and provideseven more improved magnetic properties. Thus, Ni content is preferably0.03 to 1.5 mass %.

Sn, Sb, Cu, P, Mo and Cr are elements useful to improve the magneticproperties, respectively. However, if any of these elements is containedin an amount less than its lower limit described above, it is lesseffective to improve the magnetic properties, whereas if present in anamount equal to or less than its upper limit described above, it givesthe best growth of secondary recrystallized grains. Thus, each of theseelements is preferably present in an amount within the above-describedrange. The balance other than the above-described elements is Fe andincidental impurities that are incorporated during the manufacturingprocess.

Then, the slab having the above-described chemical composition issubjected to heating before hot rolling in a conventional manner.However, the slab may also be subjected to hot rolling directly aftercasting, without being subjected to heating. In the case of a thin slab,it may be subjected to hot rolling or proceed to the subsequent step,omitting hot rolling.

Further, the hot rolled sheet is optionally subjected to hot rolledsheet annealing. A main purpose of the hot rolled sheet annealing is toimprove the magnetic properties by dissolving the band texture generatedby hot rolling to obtain a primary recrystallization texture ofuniformly-sized grains, and thereby further developing a Goss textureduring secondary recrystallization annealing. As this moment, to obtaina highly-developed Goss texture in a product sheet, a hot rolled sheetannealing temperature is preferably 800° C. to 1100° C. If a hot rolledsheet annealing temperature is lower than 800° C., there remains a bandtexture resulting from hot rolling, which makes it difficult to obtain aprimary recrystallization texture of uniformly-sized grains and impedesa desired improvement of secondary recrystallization. On the other hand,if a hot rolled sheet annealing temperature exceeds 1100° C., the grainsize after the hot rolled sheet annealing coarsens too much, which makesit difficult to obtain a primary recrystallization texture ofuniformly-sized grains.

After the hot rolled sheet annealing, the sheet is preferably subjectedto cold rolling once, or twice or more with intermediate annealingperformed therebetween, to be finished to a final sheet thickness. Thesheet is subjected to subsequent decarburization (combined withrecrystallization annealing). Then, an annealing separator is applied tothe sheet. After the application of the annealing separator, the sheetis subjected to final annealing for purposes of secondaryrecrystallization and formation of a forsterite film. It should be notedthat the annealing separator is preferably composed mainly of MgO toform forsterite. As used herein, the phrase “composed mainly of MgO”implies that any well-known compound for the annealing separator and anyproperty improvement compound other than MgO may also be containedwithin a range without interfering with the formation of a forsteritefilm intended.

After the final annealing, it is effective to subject the sheet toflattening annealing to correct the shape thereof. Insulation coating isapplied to the surfaces of the steel sheet before or after theflattening annealing. As used herein, this insulation coating means suchcoating that may apply tension to the steel sheet to reduce iron loss(hereinafter, referred to as tension coating). Tension coating includesinorganic coating containing silica and ceramic coating by physicalvapor deposition, chemical vapor deposition, and so on.

The grain oriented electrical steel sheet after final annealing ortension coating as mentioned above is subjected to magnetic domainrefining by irradiating the surfaces of the steel sheet with an electronbeam. The degree of vacuum during the electron beam irradiation may becontrolled as mentioned above to make full use of the thermal strainapplication effect through the electron beam irradiation, whileminimizing damage to the film.

Except the above-mentioned steps and manufacturing conditions, it ispossible to apply a conventionally known method of manufacturing a grainoriented electrical steel sheet where magnetic domain refining treatmentis performed by an electron beam.

Experiment 1

Steel slabs, each having a chemical composition containing the followingelements, were manufactured by continuous casting: C: 0.08 mass %; Si:3.1 mass %; Mn: 0.05 mass %; Ni: 0.01 mass %; Al: 230 mass ppm; N: 90mass ppm; Se: 180 mass ppm; S: 20 mass ppm; O: 22 mass ppm; and thebalance being Fe and incidental impurities. Then, each of these steelslabs was heated to 1400° C., subjected to hot rolling to be finished toa hot-rolled sheet having a sheet thickness of 2.0 mm, and thensubjected to hot rolled sheet annealing at 1100° C. for 120 seconds.Subsequently, each steel sheet was subjected to cold rolling to anintermediate sheet thickness of 0.65 mm, and then to intermediateannealing under the following conditions: degree of oxidationPH2O/PH2=0.32, temperature=1000° C., and duration=60 seconds.Subsequently, each steel sheet was subjected to hydrochloric acidpickling to remove subscales from the surfaces thereof, followed by coldrolling again to be finished to a cold-rolled sheet having a sheetthickness of 0.23 mm.

Then, each steel sheet was subjected to decarburization where it wasretained at a degree of oxidation PH2O/PH2 of 0.50 and a soakingtemperature of 830° C. for 60 seconds. Then, an annealing separatorcomposed mainly of MgO was applied to each steel sheet. Thereafter, eachsteel sheet was subjected to final annealing for the purposes ofsecondary recrystallization, formation of a forsterite film andpurification under the conditions of 1200° C. and 30 hours. Then, aninsulation coating composed of 60% colloidal silica and aluminumphosphate was applied to each steel sheet, which in turn was baked at800° C. This coating application process also serves as flatteningannealing.

Thereafter, one side of each steel sheet was subjected to magneticdomain refinement treatment where it was irradiated with electron beamat irradiation width of 0.15 mm and irradiation interval of 5.0 mm in adirection perpendicular to the rolling direction. Then, each steel sheetwas evaluated for magnetic properties as a product. The primaryrecrystallization annealing temperature was varied to obtain materials,each having a value of magnetic flux density B8 of 1.90 to 1.95 T. Inaddition, an electron beam was irradiated under different conditionswith different beam current values and beam scanning rates. Then, eachproduct was subjected to oblique shearing to be assembled into athree-phase transformer at 500 kVA, and then measured for its iron lossand noise in a state where it was excited at 50 Hz and 1.7 T. Thistransformer has design values of iron loss and noise of 55 dB and 0.83W/kg, respectively. The above-mentioned measurement results on iron lossand noise are shown in Table 1.

TABLE 1 Magnetic Magnetic Degree domain domain Tension of vacuumdiscontinuous discontinuous in furnace during portion on portion onduring electron strain- non-strain-i flattening beam introducedntroduced Material annealing irradiation side side Transformer IDW_(17/50)(W/kg) B_(s) (T) (MPa) (Pa) Wa/Wb (μm) (μm) W_(17/50)(W/kg)Noise (dBA) Remarks 1 0.67 1.93 22 0.05 0 290 450 0.80 62 ComparativeExample 2 0.67 1.93 16 0.05 0.2 280 440 0.80 62 Comparative Example 30.67 1.93 15 1.0 0.3 260 420 0.80 61 Comparative Example 4 0.67 1.93 92.0 0.6 250 360 0.81 55 Example of Present Invention 5 0.67 1.93 9 1.50.8 220 320 0.80 54 Example of Present Invention 6 0.73 1.93 9 3.0 0.8350 270 0.85 54 Comparative Example 7 0.67 1.93 11 2.5 1.0 190 270 0.8154 Example of Present Invention 8 0.76 1.93 11 1.5 1.0 120 200 0.91 54Comparative Example 9 0.71 1.93 11 1.5 1.0 220 200 0.86 54 ComparativeExample 10 0.73 1.93 17 0.03 0 290 550 0.88 62 Comparative Example 110.73 1.90 11 1.5 0.9 200 300 0.89 60 Comparative Example 12 0.71 1.91 92.5 0.9 200 300 0.87 59 Comparative Example 13 0.69 1.92 7 3.0 0.9 200300 0.82 55 Example of Present Invention 14 0.67 1.93 7 1.5 0.9 200 3000.81 54 Example of Present Invention 15 0.66 1.94 11 2.5 0.9 200 3000.80 53 Example of Present Invention 16 0.65 1.95 11 2.5 0.9 200 3000.79 53 Example of Present Invention

As shown in Table 1, each grain oriented electrical steel sheet, whichwas subjected to magnetic domain refining treatment by an electron beamand falls within our range, produces low noise when assembled as anactual transformer and inhibits degradation in iron loss properties. Theresulting iron loss and low noise properties are consistent with thedesign value.

In contrast, Comparative Examples of steel sample IDs 11 and 12, whichare outside our range in terms of their magnetic flux densities, allfailed to show either low noise properties or low iron loss properties.In addition, Comparative Examples of steel sample IDs 1 to 3 and 10,each of which has a value of (Wa/Wb) less than 0.5, did not offer lownoise properties. Further, Comparative Examples of steel sample IDs 6, 8and 9, which are outside our range in terms of the average width of amagnetic domain discontinuous portion in a surface of the steel sheeteither on the strain-introduced side or non-strain-introduced side,proved to exhibit inferior iron loss properties.

1. A method of manufacturing a grain oriented electrical steel sheetcomprising: subjecting a slab to rolling to a final sheet thickness;subjecting the sheet to subsequent decarburization; applying anannealing separator composed mainly of MgO to a surface of the sheetbefore subjecting the sheet to final annealing; subjecting the sheet tosubsequent tension coating; and subjecting, after the final annealing orthe tension coating, the sheet to magnetic domain refining treatment byelectron beam irradiation, wherein (1) a degree of vacuum during theelectron beam irradiation is 0.1 to 5 Pa, and (2) a tension to beexerted on the steel sheet during flattening annealing is 5 to 15 MPa.2. The method according to claim 1, wherein the slab for the grainoriented electrical steel sheet is subjected to hot rolling and,optionally, hot rolled sheet annealing, and subsequently subjected tocold rolling once, or twice or more with intermediate annealingperformed therebetween, to be finished to a final sheet thickness.